The present invention relates to a spinal disc prosthesis to replace a damaged or degenerated spinal disc in a spinal column of a human.
U.S. Pat. Nos. 5,017,437 and 5,534,030 disclose typical spinal disc prostheses to replace a damaged or degenerated spinal disc in a spinal column of a human. The discs disclosed in these patents include a pair of rigid plates adhered to opposite surfaces of a body of elastomeric material.
The disc when in use is positioned between adjacent vertebrae. The disc is subject to forces which act in the spine including compression forces due to loads on the spine, tension forces due to bending of the spine, and torsional forces due to twisting of the spine. These forces can be applied simultaneously to the disc. These forces tend to cause relative motion between the disc and the adjacent vertebrae.
The discs disclosed in these patents have spaced projections for location in cavities in the adjacent vertebrae. The projections when located in a cavity resist a tendency of the disc to move relative to the vertebrae. Thus, the projections help maintain the disc in position between vertebrae.
The cavities which receive the projections are formed in the vertebrae by the surgeon. The deeper the cavity, the more difficult it is for the surgeon to form the cavity. It is desirable to minimize the depth of the cavity and yet maintain the resistance to relative movement of the disc and vertebrae. In addition, it is desirable for the surgeon to be able to move the disc relative to the vertebrae, at least by a small amount, when the disc is located between the vertebrae. The relatively long, spaced projections of the discs disclosed in U.S. Pat. Nos. 5,017,437 and 5,534,030 require relatively deep cavities and do not readily permit the surgeon to be able to move the disc relative to the vertebrae.
The present invention relates to projections on a spinal disc which projections are received in cavities in the vertebrae between which the disc is located. Each vertebrae has one cavity which receives a respective projection on the disc. The projection and cavity have substantial area contact which minimizes the possibility of the disc being expelled upon relative movement of the vertebrae between which the disc is located. Also, the height of the projection is minimized in order to minimize the depth of the cavity, thus minimizing the amount of distraction of the vertebrae which is necessary when the cavity which is to receive a projection is formed.
Moreover, the spinal discs of the prior art have lobes projecting from the posterior portion of the disc. These lobes have been found to be areas of stress concentration where forces act to separate the rigid plates from the body of elastomeric material. Elimination of these lobes to minimize the stress concentration has been found to be desirable.